Building expects openssl/engine.h despite engine API being deprecated?

[polarathene]

Equivalent of alexcrichton/ssh2-rs#328

  cargo:warning=curl/lib/vtls/openssl.c:103:10: fatal error: 'openssl/engine.h' file not found
  cargo:warning=  103 | #include <openssl/engine.h>
  cargo:warning=      |          ^~~~~~~~~~~~~~~~~~
  cargo:warning=1 warning and 1 error generated.

---

Curl likewise can opt-out:

https://github.com/curl/curl/blob/curl-8_9_1/lib/vtls/openssl.c#L100-L104

#if (OPENSSL_VERSION_NUMBER >= 0x0090700fL) && /* 0.9.7 or later */     \
  !defined(OPENSSL_NO_ENGINE) && !defined(OPENSSL_NO_UI_CONSOLE)
#define USE_OPENSSL_ENGINE
#include <openssl/engine.h>
#endif

For Fedora 41 onwards openssl-devel-engine provides build compatibility for the time being. A brief overview follows:

https://discussion.fedoraproject.org/t/f41-change-proposal-openssl-deprecate-engine-system-wide

Engines are not FIPS compatible and corresponding API is deprecated since OpenSSL 3.0. The engine functionality we are aware of (PKCS#11, TPM) is either covered by providers or will be covered soon.

We are going to prevent creating the new packages dependent on OpenSSL ENGINE API and remove ENGINE dependencies from the existing packages.

OpenSSL libcrypto.so exports the same ENGINE_* symbols as for f40. Applications relying on the ENGINE API can’t be built but still work.